388 ME. a. J. BOMANES ON PHTSIOLO&ICAL SELECTION. 



by their occurrence to induce the primary distinction of sterility. 

 My theory, on the other hand, inverts this order, and supposes 

 the primary distinction to be likewise, as a rule, the primordial 

 distinction. Now, the advantages thus gained are two-fold. In the 

 first place, as just shown, we are able to release the principles of 

 natural selection from what appears to me the otherwise hopeless 

 difficulty of effecting diversification of specific character on an 

 overcrowded area, with nothing to prevent free intercrossing ; 

 and, in the next place, as we can now see, we are able to find an 

 additional reason for the diversification of character, over and 

 above the one that is relied upon by Mr. Darwin. For, by regard- 

 ing the primary distinction of sterility as likewdse the primordial 

 distinction, we are able to apply to an incipient variety, inhabit- 

 ing even an overcrowded area, the same principles which are 

 known to lead to diversification on oceanic islands, &c., as pre- 

 viously explained. Moreover, from any initial variation on the 

 part of the reproductive system, we should be prepared to expect 

 variations to occur in other parts of the progeny. Thus, if once we 

 regard the primary distinction of sterility as also the initial di- 

 stinction, instead of an incidental result of secondary distinctions, 

 Mr. Darwin's argument iouching the causes of diversification is 

 not merely saved : it is notably extended by the addition of two 

 independent principles which, as we know from other evidence, 

 are principles of high importance in this respect. 



AeGUMENT FEOM GrEOGEAPHIOAL DiSTETBUTION. 



From the nature of the case, there is only one other line of 

 evidence open whereby to substantiate the theory of physiologi- 

 cal selection, namely, the evidence which is afforded by the geo- 

 graphical distribution of species. But the evidence here is both 

 abundant in quantity and, to my mind, most cogent in quality. 

 On the present occasion, however, I can only give a brief sketch 

 of its main outlines. 



Mr. Darwin has adduced very good evidence to show that large 

 areas, notwithstanding the disadvantages which (on his theory) 

 must arise from free intercrossing, are what he terms better 

 manufactories of species than smaller areas, such as oceanic 

 islands. On the other hand, I have previously noticed that 

 oceanic islands are comparatively rich in peculiar species. But 

 these two statements are not incompatible. Smaller areas are, 

 as a rule, rich in peculiar species relatively to the number of 



